ADSL Systems - An Overview

1
ADSL Systems - An Overview

• Praveen Reguraman
• InterOperability Lab
• University of New Hampshire
• 1999

2
Contents

• ADSL - An Overview
• ADSL Network Architecture
• ADSL Protocol Architecture
• Typical ADSL Implementations
• RFC1483
• PPP Over ATM Over ADSL
• PPP over Ethernet.

3
ADSL

• An Overview

4
ADSL

• ADSL - Asymmetric Digital Subscriber Line
• Copper based High speed network access
  technology
• rapidly growing broadband access solution for
  home networking and small business systems
• uses multi-carrier modulation over unused
  frequency bands in phone lines
• supports data rates up to 6144 Kbps downstream
  and 640 Kbps upstream

5
ADSL Network Architecture
ATU-R
DSLAM
ATM Switch
Network Termination
Broadband network

• ATU-R - ADSL Transceiver Unit at remote end.
• DSLAM - Digital Subscriber Line Access
  Multiplexer

6
ADSL Protocol Architecture
Upper layer Protocols
Upper layer Protocols
IP
IP
PPP
PPP
AAL5
AAL5
ATM
ATM
ADSL
Internet or outside n/w
session management
Telco CO
DSL client
7
ADSL Implementations

• RFC 1483
• PPP Over ATM Over ADSL
• PPP Over Ethernet

8
RFC 1483

• RFC 1483 - Multi-Protocol Over AAL5
• creation and use of PVCs for data transfer.
• uses LLC/SNAP Encapsulation for routed/bridged
  PDUs.
• Supports IEEE 802.3/802.2, Ethernet II Bridged
  PDUs and PPP protocol units for ADSL.

9
LLC/SNAP Encapsulation

• A Brief overview

10
Payload format for routed ISO PDUS
LLC 0xFE-FE-03
ISO PDUs

• The routed/bridged PDUs are encapsulated in the
  IEEE 802.2 LLC header.
• The value 0xFE-FE-03 indicates a routed ISO PDU.
• Routed ISO protocol is identified by one octet
  NLPID field which is a part of protocol data
• IP, though has a defined NLPID, should be
  encapsulated using SNAP.
• LLC Header is followed by a SNAP header for all
  non-ISO protocols and for bridged PDUs.

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SNAP Header
OUI
PID

• The Value 0xAA-AA-03 in LLC header indicates the
  presence of a SNAP header.
• OUI - Organizationally Unique Identifier ( 3
  octets) identifies an organization which
  administers the protocol encapsulated.
• PID - Protocol Identifier ( 2 octets) identifies
  the protocol encapsulated.

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Payload format for Routed Non-ISO PDUs
LLC 0xAA-AA-03
OUI
EtherType (2 octets)
Non-ISO PDU

• The OUI Value 0x00-00-00 indicates that the
  following PID is an EtherType.
• The EtherType value is 0x08-00 for IP.

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Payload format for Bridged PDUs
LLC 0xAA-AA-03
OUI 0x00-80-C2
PID
PAD
MAC destination address
Remainder of MAC frame
LAN FCS

• The type of the bridged media is specified by the
  PID field( 2 octets).
• Padding is added to align the user information
  filed of the bridged PDU to a 4 octet boundary.

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VC Multiplexing

• VC Multiplexing is an alternative approach to
  LLC/SNAP encapsulation.
• VC Multiplexing is used when a large number of
  VCs could be dynamically created and used.
• VC Multiplexing assigns a particular VC for a
  particular protocol and all PDUs of that protocol
  use the assigned VC.
• VC multiplexing is preferred in a SVC environment
  and LLC/SNAP is used for PVCs.

15
PPP over ATM over ADSL

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PPP Over AAL5

• Uses LLC/SNAP encapsulation for PPP PDUs.
• The NLPID value is 0xCF for PPP.
• The PPP PDU contains the PPP -Protocol Identifier
  followed by information and the padding.

17
PPP - An Introduction

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Protocol summary

• PPP -designed for simple links to transport
  packets between two peers.
• PPP encapsulation provides for multiplexing of
  different network-layer protocols simultaneously
  over the same link.
• PPP provides a Link Control Protocol (LCP) which
  negotiates the establishment and termination of a
  PPP link.
• LCP also negotiates the options for encapsulation
  format,authentication and link quality
  monitoring.
• The network layer protocols that are supported by
  the PPP link are managed by a family of network
  control protocols defined.

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PPP Encapsulation
Protocol (8/16 bits)
Information
Padding

• The Protocol field identifies the datagram
  encapsulated in the information field.
• Information filed contains the datagram and could
  be zero or more octets up to a Maximum Receive
  Unit (MRU).
• The default MRU value is 1500 octets ,though it
  could vary by negotiation.
• The Information field may be padded up to the MRU
  number of octets.

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PPP Link Operation.
LCP
PAP/CHAP
UP
Dead
Establishment
Authentication
fail
fail
LCP
closing
NCP
Termination
Network
down

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Link Establishment

• Link establishment phase uses the Link control
  protocol.
• Link Configuration Options
  - the Maximum
  Receive Unit size.
  - Authentication and protocol to be used
  for authentication - Protocol
  Field Compression.
  - Link quality monitoring
  -
  Magic number option for detecting looped back
  links. - Address and Control field
  compression.
  

22
Link Establishment Process
Configure request
Peer
Peer
Configure Ack/Nak

• The configure-request message is sent to request
  a link establishment and it contains the various
  options requested.
• This request is responded with a Configure-Ack if
  the negotiation is accepted.
• A Configure-Nak is sent if the negotiation is not
  acceptable and it suggests an acceptable
  negotiation.

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Authentication

• Authentication Option uses Password
  Authentication Protocol (PAP) or Challenge
  Handshake Authentication Protocol (CHAP).
• The protocol used depends on negotiation.
• CHAP uses a one-way hashing algorithm which is
  known only to the user, to respond to a challenge
  sent by the authenticator.
• CHAP is more secure than PAP.

24
Magic Number

• This option detects looped back links.
• The magic number is a unique number chosen in the
  most random way possible.
• This number enabled in a configure request is
  compared with the last request sent and if equal
  indicate a looped back link.
• Link quality monitoring also uses this option to
  verify proper link communication.

25
PPP Over Ethernet

• A Brief Introduction.

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PPPoE - How it works

• PPPoE - enables establishing PPP sessions and
  encapsulating PPP packets over Ethernet.
• It has two distinct stages - A discovery stage
  and a Session stage.
• The discovery stage identifies the Ethernet MAC
  address of the peer with which the PPP session is
  to be established.
• The session stage gets a PPPoE session ID for the
  PPP session and encapsulates PPP packets.

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Discovery stage

• The discovery stage works similar to the ARP
  (Address resolution) request-response mechanism.
• The host which is required to establish a PPP
  session broadcasts a session Initiation packet.
• The Initiation packet is responded with offer
  packets from the Access Concentrators accepting
  the request.
• The host follows with a unicast session request
  packet to a particular Access concentrator.
• The Access concentrator replies with a
  confirmation and a session ID is obtained and the
  session established.

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Ethernet Payload for PPPoE
VER
TYPE
CODE
SESSION_ID
LENGTH
payload

• The EtherType is set to 0x8863 for discovery
  stage and 0x8864 for PPP session stage.
• PPPoE provides the ability to connect a network
  of hosts over a bridging access device to a
  Remote access concentrator.

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Conclusion

• ADSL is rapidly gaining widespread attention due
  to its low cost broadband access solution .
• Various methods are developed to enable the mass
  deployment of ADSL systems .
• With QoS metrics gaining more attention day by
  day, the impact of QoS implementations on the
  cost effectiveness of ADSL solutions is an
  interesting thought.
• How the existing solutions adapt themselves to
  QoS metrics or how they should be modified will
  lead to interesting research.